It is conventional practice in the art of electro-hydraulic servo control systems to provide a command signal indicative of position, velocity, acceleration or pressure desired at the controlled mechanism, to measure actual position, velocity and acceleration of the controlled mechanism by means of corresponding transducers, and to drive a hydraulic actuator with an error signal representative of a difference between the command signal and the measured motion variables. Provision of three transducers mounted on or otherwise responsive to the controlled mechanism increases significantly the overall expense of the servo system while at the same time reducing overall reliability. The aforementioned deficiencies in the art are particularly acute in the field of industrial robotics where interest in cost, simplicity and reliability is continually increasing.
Barker, "Design of a State Observer For Improving the Response of a Linear Pneumatic Servo-Actuator", Fluids in Control and Automation, Paper C5 (1976) discusses design theory for an observer for estimating unmeasurable state variables in a pneumatic servo-actuated guidance control system. In addition to a mathematical analysis of the first order system state functions and eigenvalue assignment theory, the paper discloses a specific analog observer control system wherein a pneumatic servo actuator is controlled as a combined function of the actuator command signal, measured actuator position, and actuator velocity and acceleration estimated by the observer.